Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a communications network, sometimes also referred to as a wireless communication system, a cellular communications network, a cellular radio system or a cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more Core Networks (CN), comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB” or “B node” depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the user equipment. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the user equipment to the base station.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller may supervise and coordinate various activities of the plural base stations connected thereto.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE is controlled by the radio base station.
Overall E-UTRAN Architecture
The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) comprises of base stations called enhanced NodeBs (eNBs or eNodeBs). These base stations provide the protocol terminations for a user plane and a control plane towards the UE in the E-UTRAN. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the Evolved Packet Core (EPC), more specifically to the Mobility Management Entity (MME) by means of the S1-MME interface and to the Serving Gateway (S-GW) by means of the S1-U interface. The S1 interface supports many-to-many relations between the MMES and the eNBs and between the S-GWs and the eNBs. The E-UTRAN architecture is illustrated in FIG. 1.
The eNB hosts functionalities such as Radio Resource Management (RRM), radio bearer control, admission control, header compression of user plane data towards the serving gateway, routing of user plane data towards the serving gateway. The MME is the control node that processes the signaling between the UE and the CN. The main functions of the MME are related to connection management and bearer management, which are handled via Non Access Stratum (NAS) protocols. The S-GW is the anchor point for UE mobility. The S-GW also includes other functionalities such as temporary DL data buffering while the UE is being paged, packet routing and forwarding the right eNB, gathering of information for charging and lawful interception. The PDN Gateway (P-GVV) is the node responsible for allocation of an Internet Protocol (IP) address of the UE. The PDN Gateway is also responsible for Quality of Service (QoS) enforcement, which is explained further below. FIG. 2 shows the functional split between E-UTRAN and EPC and gives a summary of the functionalities of the different nodes. The reader is referred to 3GPP TS 36.300 and the references therein for the details of the functionalities of the different nodes. In FIG. 2, the outer larger boxes depict the logical nodes; the boxes inside the outer larger boxes depict the functional entities of the control plane. The dashed boxes inside the outer eNB box depict the radio protocol layers.
OAM Architecture
A management system for managing the radio access network is shown in FIG. 3. Node Elements (NE), also referred to as eNodeB in FIG. 3 or eNB throughout the document, are managed by a Domain Manager (DM), also referred to as the Operation and Support System (OSS). A DM may further be managed by a Network Manager (NM). Two NEs are interfaced by X2, whereas the interface between two DMs is referred to as Itf-P2P. The management system may configure the NEs, as well as receive observations associated to features in the NEs. For example, the DM observes and configures the NEs, while the NM observes and configures the DM, as well as the NE via the DM.
It is further assumed that any feature that automatically optimizes parameters used by the NE may in principle be executed in the NE, DM, or the NM. Such features are referred to as Self-Organizing Network (SON) features.
Radio Protocol Architecture
The radio protocol architecture of E-UTRAN is divided into the user plane and the control plane.
User Plane
FIG. 4 shows a protocol stack for the user-plane comprising different layers. The protocol stack for the user plane is comprised of the Packet Data Convergence Protocol (PDCP) layer, the Radio Link Control (RLC) layer, and the Medium Access Control (MAC) layer. The PDCP layer, the RLC layer and the MAC layer are terminated at the eNB. The PDCP manages IP packets in the user plane and it performs functionalities such as header compression, security, and re-ordering and retransmission during handover. The RLC layer is mainly responsible for segmentation and corresponding assembly of PDCP packets, such that the size of the PDCP packets fit a size for a packet that is actually to be transmitted over the air interface. The RLC may operate either in unacknowledged mode or acknowledged mode, where the latter supports retransmissions. The MAC layer performs multiplexing of data from different radio bearers. The MAC layer informs the RLC layer about the size of the Internet Protocol (IP) packets to provide, which is decided based on the required QoS of each radio bearer and a current capacity available to the UE.
Control Plane
FIG. 5 shows the protocol stack of the control plane. The layers below the Radio Resource Control (RRC) layer perform the same functionality as in the user plane except that there is no header compression in the control plane. The main functions of the RRC are broadcasting of system information, controlling an RRC connection, i.e. establishment, modification, and release of the RRC connection, establishment of Signalling Radio Bearers (SRBs) and Data Radio Bearers (DRBs), handover, configuration of lower protocol layers, radio link failure recovery, and measurement configuration and reporting. The details of the RRC protocol functionalities and procedures may be found in 3GPP TS 36.331.
The UE is uniquely identified over the S1 interface within an eNB with an Information Element (IE) named “eNB UE S1AP ID”. When an MME receives the eNB UE S1AP ID it stores it for the duration of the logical S1-connection associated with the UE. Once the MME has identified the UE, the eNB UE S1AP ID IE is included in all UE associated S1-AP signalling. The eNB UE S1AP ID is unique within the eNB. After a handover, the UEs are assigned a new S1AP ID by a target eNB.
From the MME side, the UE is uniquely identified using the MME UE S1AP ID IE. When the eNB receives the MME UE S1AP ID the MME stores the MME UE S1AP ID for the duration of the logical S1 connection associated with the UE. Once known to an eNB this MME UE S1AP ID IE is included in all UE associated S1-AP signalling. The MME UE S1AP ID is unique within the MME, and it is changed if the UE connects to a new MME. This may for example happen after the handover between two eNBs connected to different MMEs.
Flow of User Plane and Control Plane Data
A flow of user plane and control plane data is illustrated in FIG. 6. There is only one MAC entity per UE, unless the UE supports multiple carriers as in the case of carrier aggregation. Under this MAC entity, several Hybrid Automatic Repeat reQuest (HARQ) processes may be running simultaneously for rapid retransmissions. There is a separate RLC entity for each radio bearer and if the radio bearer is configured to use PDCP, there is also one separate PDCP entity for that bearer. The bearer is configured to use PDCP only if the bearer is dedicated to a UE, i.e. multicast and broadcast data do not utilize PDCP both in the control and user plane and the PDCP is used only for dedicated control message in the control plane and for dedicated UL/DL data in the user plane.
With respect to the protocol stack shown in FIG. 2, each layer receives a Service Data Unit (SDU) from a higher layer, and sends a Protocol Data Unit (PDU) to the lower layer. For example, a PDCP PDU is sent towards the RLC, and the PDCP PDU is a RLC SDU from the point of view of the RLC. The RLC in turn sends an RLC PDU towards the MAC. The RLC PDU is a MAC SDU from the point of view of the MAC. In the opposite direction, the process is reversed, i.e. each layer passes the SDUs to the layer above, where the SDUs are perceived as PDUs.
Heterogeneous Networks and Dual/Multiple Connectivity
The use of a so called heterogeneous deployment or a heterogeneous network is illustrated in FIG. 7. It comprises of transmission nodes with different transmit power and with overlapping coverage areas. In a typical case, there may be multiple pico nodes within the coverage area of a macro node. The heterogeneous deployment is considered to be an interesting future deployment strategy for cellular networks. In such a deployment, the low-power nodes, hereafter also denoted as “pico nodes” are typically assumed to offer high data rates, Mbit/s, as well as provide high capacity, users/m2 or Mbit/s/m2, in the local areas where it is needed and/or desired The high-power nodes, hereafter also denoted as “macro nodes” are assumed to provide full-area coverage. In practice, the macro nodes may correspond to currently deployed macro cells while the pico nodes may be later deployed nodes, extending the capacity and/or achievable data rates within the macro-cell coverage area where needed.
A pico node in a heterogeneous deployment may correspond to a cell of its own, a “pico cell”, see FIG. 8. This means that, in addition to downlink and uplink data transmission and/or reception, the pico node also transmits the full set of common signals and channels associated with a cell. In the LTE context this includes:                The Primary and Secondary Synchronization Signals (PSS and SSS) corresponding to a Physical Cell Identity (PCI) of the pico cell.        The Cell-specific Reference Signals (CRS), also corresponding to the PCI of the cell. The CRS may e.g. be used for downlink channel estimation to enable coherent demodulation of downlink transmissions.        The Broadcast CHannel (BCH), with corresponding pico-cell system information. As the pico node transmits the common signals and channels, the corresponding pico cell may be detected and selected, i.e. connected to, by the UE.        
If the pico node corresponds to a cell of its own, also so-called L1/L2 control signaling on the Physical Downlink Control Channel (PDCCH), as well as on the Physical Control Format Indicator Channel (PCFICH) and on the Physical Hybrid-ARQ Indicator Channel (PHICH), is transmitted from the pico node to the connected UEs, in addition to downlink data transmission on the Physical Downlink Shared Channel (PDSCH). The L1/L2 control signaling provides for example downlink and uplink scheduling information and HARQ-related information to UEs within the cell. This is shown in FIG. 8. The indices “p” and “m” in FIG. 8 indicate common signals/channels for the pico and macro cell respectively.
Alternatively, a pico node within a heterogeneous deployment may not correspond to a cell of its own but may just provide a data-rate and a capacity extension of the overlaid macro cell. This is sometimes known as a shared cell or a soft cell. In this case at least the CRS, a Physical Broadcast CHannel (PBCH), the PSS and the SSS are transmitted from the macro node. The PDSCH may be transmitted from the pico node. To allow for demodulation and detection of the PDSCH, despite the fact that no CRS is transmitted from the pico node, Demodulation Reference Signals (DM-RS) should be transmitted from the pico node together with the PDSCH. The UE-specific reference signals may then be used by the UE for demodulation and detection of the PDSCH. This is shown in FIG. 9 where the pico node does not correspond to a cell of its own.
Transmitting data from a pico node which do not transmit the CRS as described above requires support for DM-RS in the UE. A UE that supports DM-RS may be referred to as a non-legacy terminal. In LTE, DM-RS-based PDSCH reception is supported in Rel-10 and for Frequency Division Duplex (FDD). For the L1/L2 control signalling, DM-RS-based reception is planned for Rel-11. For UEs not supporting DM-RS-based reception, i.e. legacy UEs one possibility in a shared cell deployment is to exploit transmissions with a Single Frequency Network (SFN)—or a similar type of transmission. In essence identical copies of the signals and channels necessary for a legacy UE are transmitted simultaneously from the macro and pico nodes. From a perspective of the UE this will look as a single transmission. An SFN operation with identical transmission from macro and pico to a terminal, which is illustrated in FIG. 10, will only provide a Signal-to-Interference-plus-Noise Ratio (SINR) gain, which may be translated into a higher data rate but not into a capacity improvement. This is because transmission resources may not be reused across eNBs within the same cell.
If we may assume that the macros are able to provide coverage and the picos are there only for capacity enhancements, i.e. there are no coverage holes, another alternative architecture is where the UE maintains the connection with the macro node, called the “anchor” link, all the time, and adds a connection to the pico node, referred to as the booster link, when it is in the coverage area of the pico node. This is known as a dual connectivity and the idea may be extended to a multiple connectivity where the UE may be connected to one anchor node and several booster nodes. When both connections are active, the anchor link may be used for control signalling while the booster link is used for data. However, it will still be possible to send data also via the anchor link. Dual connectivity operation with the UE having simultaneous active connections with both the anchor and booster is illustrated in FIG. 11. Note that in this case, as in the previous cases, the system information is shown to be sent only from the macro nodes, but it is still possible to send it also from the pico nodes.
Wireless communications networks are becoming more flexible, for example with the use of heterogeneous networks. Such flexible networks also present problems, such as difficulty to serve UEs in proximity of two cells with good quality in both DL and UL due to the properties of such flexible networks. Such properties may be related to different transmission powers of base stations in the network. Flexible networks, such as heterogeneous networks, may offer higher capacity to user equipments by using dual connectivity, but sometimes at the cost of lower radio resource utilization.